It is a UPS topology offering middle ground between offline (stand by) and double-conversion on-line uninterruptible power supply. They are equipped with AVR (Automatic Voltage Regulation) which is used to maintain the output voltage within an acceptable range. This is achieved by switching the taps on the transformer, based on the value of the incoming voltage to either buck or boost it. Technology is also known as VI (Voltage Independent), regulates voltage but not frequency.
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